This invention relates to the detection and mapping of internal stresses in the interior of bulk materials by scanning acoustic technique.
It is known to utilize an ultrasonic microscope to compare a plurality of images detected by the ultrasonic waves reflected on or passing through a sample under different conditions by displaying the images on a cathode ray tube, such as shown in U.S. Pat. No. 4,674,333. It is also known to use a scanning acoustic microscope (SAM) for inspection and quality control in manufacturing and other industrial applications wherein the object under investigation is insonified by ultrasonic acoustic pulses, and ultrasonic reflections from the object are received and utilized to generate electrical signals which are used to develop an image of the object, the device being capable of focusing on varying transition levels within the object, as shown in U.S. Pat. No. 4,866,986. It is known from this latter patent that when an acoustic pulse encounters any discontinuity (change of the acoustic impedance of the medium through which it is traveling), part of its energy is reflected and it is these echoes that a reflection mode acoustic microscope receives and eventually employs to display an image of internal features of the target object. Various types of information are present in the returned or echo pulses. For example, the time delay between radiation and reception provides an accurate index of the depth, or distance in the direction of travel, of a discontinuity.
The use of acoustic microscopy for nondestructive examination of materials is also shown in the following U.S. Pat. Nos. 4,531,410; 4,702,112; and 4,788,866. These are incorporated herein by reference.
The use of acoustic microscopy for nondestructive testing of internal physical characteristics of bodies of metal and ceramic materials is also described in co-pending U.S. patent application Ser. No. 07/922,845, filed Jul. 31, 1992, in which the applicant is a joint inventor.
Characteristic features of acoustic microscopes are also described in the following U.S. Pat. Nos.: 4,503,708; 4,459,852; and 5,079,952, for example. All of the above prior patents are incorporated herein by reference as showing known structural and functional features of acoustic microscopes used in nondestructive examination of bodies of materials.
None of the above prior art, however, teaches a method and apparatus for visualizing by acoustic microscope imaging the internal stresses in the volume of solid transparent and non-transparent to light materials.
Although acoustic birefringence is well known in the literature, the attention of researchers has been attracted to the techniques of measuring the effect of stresses on acoustic velocity. The acoustic elastic effect, or the sensitivity of velocity of particular modes to applied stresses, has been used by investigators for imaging the stress field in metals. Benson and Raelsen proposed this method and reported the experimental data of the effects of stress on acoustic velocity in simple compression (R. W. Benson and V. J. Raelson, Product. Eng. 30. Acousto-elasticity, 1959). In analogy to photoelasticity, they found birefringence to be proportional to the difference of two principal stresses in a plane specimen. Toupin and Bernstein (R. A. Toupin and B. Bernstein, J. Acoust. Soc. Am. 33, 216, 1961) derived the relations for acoustoelastic effects and determined the third order elastic constants of an isotropic material. Thurston and Brugger (R. N. Thurston and K. Brugger, Phys. Rev., Vol. A133, (1604-1610) 1964) discussed, in general, the wave propagation in a strained material. Hughes and Kelly (D. S. Hughes and J. L. Kelly, Phys. Rev. 92, 5, 1953) derived expressions for elastic wave velocities in terms of the Murnagham third order elastic constants l, m,, n, for the case of a normally isotropic body, taking the Lame constants .mu. and .lambda., as the second order coefficients. Crecraft (D. I. Crecraft, J. Sound Vib. 5, (I),(173-192), 1967) compared photoelasticity and "sonoelasticity" and presented the results of measurements of stress-induced velocity variations of both longitudinal and shear ultrasonic waves to megacycle frequencies. Data are provided for Polystyrene, Armco-Iron, Pyrex, Nickel-Steel, Copper (99.9%) and Aluminum (99%). Hsu (N. N. Hsu, Experimental Mech. Vol. 14, No. 5, (169-176), 1974) applied the pulse overlap technique for velocity measurements increasing the accuracy compared to "singaround" technique used by Crecraft. However difficulties were encountered when an attempt was made to apply acoustoelastic theory with ultrasonic measurements of residual stresses. The most recent theories (Yih-Hsing Pao, Tsung-Tsong Wu and U. Gamer, J. App. Mech. Vol. 58/11 1991) take into account the effects of plastic deformation, texture and other sources of anisotropy in materials. Recently, velocity changes were monitored by many investigators for stress imaging in metals (J. H. Cantrell and K. Salama, Intern. Mat. Rev., Vol. 36, No. 4, 1991; G. S. Kino et al., J. Appl. Phys. Vol. 50, (2607-2613), 1979; G. C. Johnson, J. Appl. Mech. Vol. 48/791, 1981; M. Hirao and Yih-Hsing Pao, J. Acoust. Soc. Am, Vol. 77 No. 5, 1985; S. W. Meeks et al. , Appl. Phys. Lett. Vol. 55, (18) 1989; J. H. Cantrell and M. Qian, Appl. Phys. Lett. Vol. 57, No. 18, 1990) and ceramics (K. F. Young, IBM J. Res. Develop. Vol. 34, No. 5, 1990).
Although the effect is small (less than 1%), many very elaborate and precise efforts were made to image velocity change on both macroscopic and microscopic scale in order to obtain the information on distribution of stresses mostly in the areas close to the surface of the sample.